The present invention relates to manufacturing scheduling, and in particular to a manufacturing scheduling system capable of deriving an optimal manufacturing schedule with respect to various tool preferences.
Semiconductor wafers are processed to produce circuits using a plurality of sequential process steps. Each step can be performed using a plurality of different processing tools. Generally, a fabrication system contains a plurality of tools, wherein tools capable of executing a certain process step are organized into one tool group. The tool can be dedicated to one tool group, or it can belong to multiple tool groups. For the tool shared by different tool groups, the more tool groups it belongs to, the greater the likelihood that it will be in use when required by another tool group. For the tool dedicated to just one tool group, it may remain idle while other tools have wafers waiting to be processed.
Referring to FIG. 1A, the manufacture of four wafer lots L1, L2, L3, and L4 corresponding respectively to products P1, P2, P3, and P4 are in the planning stage. Wafer lots L1, L2, L3 and L4 are available for operating processes OP11, OP21, OP31, and OP41, which can be executed by tool groups TG1, TG2, TG3 and TG4 respectively. The tool group TG1 contains tools A and B; tool group TG2 contains tools A, C, and D; tool group TG3 contains tools C, E, and F; and tool group TG4 contains tool E. Referring to FIG. 1B, a table 10 contains fields for wafer lot ID number (field 11) and corresponding product ID number (field 13), processing tool group ID number (field 15) and tool components thereof (field 16). After defining the tool groups and processing routes for each wafer lot, one tool is selected from each tool group for processing wafer lots L1, L2, L3 and L4 respectively. The tool selection process operates on the assumption that there is no difference between tools in a particular tool group. In other words, tools put in the same tool group have an equal chance of being selected, and are practical equivalents. If two or more wafer lots are scheduled to be processed by the same tool, the wafer lots are arranged in order according to their committed due date or other ranking algorithms. Table 10 also shows fields for the selected tool (field 17). Tools A, C, E, and E are selected for processing wafer lots L1, L2, L3 and L4 respectively. According to this example, each process takes one work day (shown in field 18 of table 10), and both wafer lots L3 and L4 are to be processed by tool E, therefore either wafer lot L3 or L4 must wait for processing until day 2. The tool utilization is shown in FIG. 1C. Among tools A to F, tools A and C are occupied on Day 1, tool E is occupied on Days 1 and 2, and tools B, D, and F are idle on Days 1 and 2. On Day 1, three out of six tools are occupied, and the utilization rate is 50%; on Day 2, one out of six tools is occupied, and the utilization rate is 17%. Obviously, the capacity of the fabrication system is not fully utilized, and the overall throughput of the fabrication system is low.
Hence, there is a need for a manufacturing system which addresses the low-utilization problems arising from the existing technology.